The presence of ventricular ectopic beats in the heart action of a patient often gives early warning of impending conditions that can be fatal. It is important, therefore, that equipment used to monitor the patient's condition provide indicia of their presence so that timely remedial action can be taken. However, it is equally important that the frequency of false indicia be reduced to a minumum in order that the time of medical attendants may be used efficiently and to prevent the presence of the indicia from being ignored. Unfortunately, however, present monitors leave much to be desired in this regard.
One method of identifying a ventricular ectopic heartbeat is by comparing its ECG wave with a wave that is considered to be normal for the particular patient involved. Any one of the number of empirical formulae can be used for this purpose, but in addition to certain physiological data, many require an input as to the measure of the degree of correlation between the form of the ECG wave for the heartbeat being examined and the form of the ECG wave representing the normal heartbeat. A reliable measurement can be attained only if the waves are effectively aligned, so that the portions of each that correspond to the same part of the heartbeat cycle are in time coincidence. This result can be attained by locating a single point in each heartbeat cycle, known as a fiducial point, that uniquely occurs at the same functional point in any ECG wave and aligning the waves with their points in time coincidence. If the location of the fiducial points is not accurate, the degree of form correlation, even for identical waves, can be far less than the actual 100 percent. In the present state of the art, the location of the fiducial points is unreliable, so that too many ectopic beats are not detected or too many false alarms are given.
In determining whether a particular heartbeat is a ventricular ectopic beat, data relating to the time between it and the previous beat, the average time between beats, and the average deviation of the beats from the average time between them is often used. If, as in the prior art, the fiducial point is not properly located, this data will be inaccurate so as to make the identification of a ventricular ectopic beat unreliable.
In making form correlation, data as to the area of non-overlap between the ECG wave being examined and the ECG wave that is considered normal is often used. In prior methods, correlation has depended on the sum of the absolute differences in the areas of corresponding samples of the wave being examined and the normal wave, so that the correlation changes with amplitude. This makes it difficult to identify the ectopic beat.